This invention relates generally to a hydraulic fluid power circuit, and more particularly to a motor vehicle hydraulic fluid power circuit having a hydraulic pump for supplying fluid to assist the operator in steering and breaking the vehicle.
Motor vehicles are frequently provided with a hydraulic pump which is driven by the engine of the vehicle to provide a source of fluid power. In passenger cars, this fluid power is frequently used to assist the operator in steering the vehicle. Because of increasing automobile braking performance requirements and because of a decreasing ability of intake manifold vacuum braking systems to provide that performance, it has been suggested to use the fluid power of the engine driven hydraulic pump to assist the operator in braking the vehicle, as well as to assist the operator in steering the vehicle.
The hydraulic pumps which are used in such systems include a relief valve which is generally internal and which connects the outlet of the pump to the inlet of the pump at a relief pressure to prevent excessive pressures in the system. If the working pressure of the brake booster reaches or exceeds the relief pressure of the pump relief valve, the pump relief valve will be actuated and the output flow from the pump will thereby be decreased to substantially zero. This terminates flow of fluid to the steering valve with the undesirable result of terminating the power steering assist for the operator until the brake booster working pressure decreases to a pressure below the relief pressure of the pump relief valve.
To limit the working pressure of the brake booster to a pressure less than the relief pressure of the pump relief valve in order to prevent this from happening, it is disclosed in the above referenced U.S. Pat. applications, Ser. Nos. 379,319 and 419,999 of James P. Baker that the brake booster power chamber can be connected to drain to prevent the working pressure of the brake booster from reaching or exceeding the pump relief pressure. Although the structure disclosed in these two United States patent applications overcomes the problem of the brake booster working pressure exceeding the pump relief pressure and thereby terminating flow to the steering valve, this structure has the disadvantage that some of the output of the pump is wasted because it is connected back to the drain line through the brake booster.